1. Field of the Invention
This invention relates generally to electrophotography, and more particularly, to an improvement in charging a predetermined portion of an electrophotographic element in a manner which creates uniform charge up to the edges of the predetermined portion.
2. Description of the Prior Art
In electrophotography, it is common to apply a uniform electrostatic charge to the surface of a recording element or film which generally consists of a photoconductive layer overlying a conductive layer. The charge is then selectively dissipated in a pattern by exposing the surface to a light image. The resulting pattern of charges produces an electrostatic latent image on the photoconductive layer which is rendered visible by applying thereto electrostatically charged developer particles which adhere to the surface of the photoconductive layer by electrostatic forces. A permanent visible image can be obtained, for example, by using developer particles which can be heat fused to the photoconductive layer, and subjecting it to a heat application step.
Charging is conventionally accomplished by exposing the surface of the photoconductive layer to a corona discharge, the polarity of which is chosen to produce the desired results upon the particular photoconductive layer being charged. Superior image reproductions are obtainable only when very uniform electrostatic charges are established on the photoconductive layer before imaging.
In many electrophotographic apparatus, either the corona generating element or the electrophotographic recording element is moved during charging, which to some extent improves uniformity of charge over the surface of the photoconductive layer. In some electrophotographic apparatus, charging takes place with no relative movement between the corona generating element and the electrophotographic recording element. In such cases, the recording element may be a multi-frame microfiche and charging is commonly restricted to a small area on the electrophotographic member by some form of shielding or masking means. This form of charging is accomplished without relative movement between the microfiche and the charging means, and it conventionally results in a generally uniform potential of several hundred volts across most of the surface being charged and a potential of zero volts at the borders of the area being charged. Unfortunately, the portion of the surface having uniformm charge does not extend up to the borders. Rather, the amount of charge tapers down to zero volts over some finite distance as the borders are approached. After imaging and developing the charged area, this border area has undesirable edge toning because of the charge gradient occuring there. Where the imaging step dissipates the entire charge at the border region, edge toning is not such a problem, but in conventional apparatus, the charge in the border region is seldom entirely dissipated.
In view of the shortcomings of the prior art, it is an object of the present invention to apply a uniform electrostatic charge to a predetermined portion of an electrophotographic film in a manner which produces uniform charge up to the edges of the portion being charged.